Finite-element modeling of thin-film damping in micromachined microphones employing diffraction based optical readout

Abstract

Micromachined microphones with diffraction-based optical displacement detection have been presented in detail previously [J. Acoust. Soc. Am. 118(5), 3000–3009 (2005)]. In addition to providing superior diaphragm displacement detection sensitivity over miniature capacitive sensors, optical approaches have the advantage of removing mechanical design constraints on the microphone backplate perforation architecture as well as constraints on the diaphragm-backplate gap thickness. Taking advantage of this freedom to tailor design the frequency response function and thermal noise characteristics of miniature high-performance microphones requires a sophisticated damping model to navigate this new design space. A finite-element model in ansys based on the modal projection method is employed to study the dynamics of new optical microphone structures. The model extracts the frequency-dependent resistance and stiffening characteristics of the film using modal displacement profiles of the diaphragm. Simulated frequency response functions and thermal-mechanical noise limits agree well with those measured on fabricated structures. Most notably, 1.5-mm-diameter diaphragm structures with under 1 μPa/√Hz thermal noise and over 20-kHz bandwidth have been successfully designed, fabricated, and characterized. [The authors would like acknowledge the IC Postdoctoral Research Fellowship Program.]